URI Encoding and Decoding

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Introduction

Here are two functions of URI encoding and decoding. They use std::string as the argument and return type.

A URI is represented as a sequence of characters, not as a sequence of octets. That is because URI might be “transported” by means that are not through a computer network, e.g., printed on paper, read over the radio, etc.—RFC2396

UriEncode() maps octets to characters, such as:

"12" -> "%00%01%02"
"~ABCD"  -> "%7EABCD"

Each octet except alphanum is converted to “% HEX HEX”. UriDecode() converts them back.

Code Snippets

Encode:

std::string UriEncode(const std::string & sSrc)
{
   const char DEC2HEX[16 + 1] = "0123456789ABCDEF";
   const unsigned char * pSrc = (const unsigned char *)sSrc.c_str();
   const int SRC_LEN = sSrc.length();
   unsigned char * const pStart = new unsigned char[SRC_LEN * 3];
   unsigned char * pEnd = pStart;
   const unsigned char * const SRC_END = pSrc + SRC_LEN;

   for (; pSrc < SRC_END; ++pSrc)
   {
      if (SAFE[*pSrc])
         *pEnd++ = *pSrc;
      else
      {
         // escape this char
         *pEnd++ = '%';
         *pEnd++ = DEC2HEX[*pSrc >> 4];
         *pEnd++ = DEC2HEX[*pSrc & 0x0F];
      }
   }

   std::string sResult((char *)pStart, (char *)pEnd);
   delete [] pStart;
   return sResult;
}

Decode:

std::string UriDecode(const std::string & sSrc)
{
   // Note from RFC1630: "Sequences which start with a percent
   // sign but are not followed by two hexadecimal characters
   // (0-9, A-F) are reserved for future extension"

   const unsigned char * pSrc = (const unsigned char *)sSrc.c_str();
   const int SRC_LEN = sSrc.length();
   const unsigned char * const SRC_END = pSrc + SRC_LEN;
   // last decodable '%'
   const unsigned char * const SRC_LAST_DEC = SRC_END - 2;

   char * const pStart = new char[SRC_LEN];
   char * pEnd = pStart;

   while (pSrc < SRC_LAST_DEC)
   {
      if (*pSrc == '%')
      {
         char dec1, dec2;
         if (-1 != (dec1 = HEX2DEC[*(pSrc + 1)])
            && -1 != (dec2 = HEX2DEC[*(pSrc + 2)]))
         {
            *pEnd++ = (dec1 << 4) + dec2;
            pSrc += 3;
            continue;
         }
      }

      *pEnd++ = *pSrc++;
   }

   // the last 2- chars
   while (pSrc < SRC_END)
      *pEnd++ = *pSrc++;

   std::string sResult(pStart, pEnd);
   delete [] pStart;
   return sResult;
}

Usage Example

Just copy the source codes or external link the functions to use them.

int main()
{
   extern std::string UriEncode(const std::string & sSrc);
   extern std::string UriDecode(const std::string & sSrc);
   const std::string ORG("12", 3);
   const std::string ENC("%00%01%02");
   assert(UriEncode(ORG) == ENC);
   assert(UriDecode(ENC) == ORG);
   return 0;
}

Differences from Other Implementations

Following are other implementations on URI/URL encoding and decoding.

• URL Encoding (by Chandrasekhar Vuppalapati): An MFC class do single char convertion.
• URLEncode (by Ryszard Krakowiak): URL encode function using MFC, convert a null terminated C string.

This implementation differs from the above in these ways because it:

• Has a decode function.
• Encodes a buffer. Also, it supports encoding a char buffer, including ‘’. Example:

"ABCABC" -> "ABC%00ABC"

• Runs faster because it uses a array to do the mapping.
• Is portable. It doesn’t use MFC CString.